Reinforced carbon nanotubes

ABSTRACT

The present invention relates generally to reinforced carbon nanotubes, and more particularly to reinforced carbon nanotubes having a plurality of microparticulate carbide or oxide materials formed substantially on the surface of such reinforced carbon nanotubes composite materials. In particular, the present invention provides reinforced carbon nanotubes (CNTs) having a plurality of boron carbide nanolumps formed substantially on a surface of the reinforced CNTs that provide a reinforcing effect on CNTs, enabling their use as effective reinforcing fillers for matrix materials to give high-strength composites. The present invention also provides methods for producing such carbide reinforced CNTs.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/347,808, filed on Jan. 11, 2002, which is hereby incorporatedherein by reference in its entirety.

GOVERNMENT SUPPORT

The present invention was made with partial support from The US ArmyNatick Soldier Systems Center (DAAD, Grant Number 16-00-C-9227),Department of Energy (Grant Number DE-FG02-00ER45805) and The NationalScience Foundation (Grant Number DMR-9996289)

FIELD OF THE INVENTION

The present invention relates generally to reinforced carbon nanotubes,and more particularly to reinforced carbon nanotubes having a pluralityof microparticulate carbide materials formed substantially on thesurface of such reinforced carbon nanotubes composite materials.

BACKGROUND OF THE INVENTION

Reinforcing fillers are usually added to a matrix material to formhigh-strength composites. In order for the resulting composites to beuseful, the reinforcing fillers must have a high load-bearing abilityand binding affinity for the matrix. Carbon nanotubes (CNTs) have beenadded to matrix materials to form high-strength composites. However, theuse of CNTs as reinforcing fillers, including multi-walled CNTs, hasseveral disadvantages. Multi-walled CNTs have a tendency to pull out of,or slip from the matrix material, resulting in reduced load bearingability. This is attributed to the fact that the interface between thematrix material and nanotube layers is very weak, thereby causing a“sword-in-sheath” type failure mechanism. Typically, only the outermostlayer of multi-wall CNTs contributes to load bearing strength. (See, forexample, D. Qian, et al. Appl. Phys. Lett., 76, 2868 (2000) and C.Bower, et al. Appl. Phys. Lett., 74, 3317 (1999)). Because of the weakvan der Waals interaction between the CNTs cylindrical graphene sheets,improved bonding between carbon nanomaterials such as relatively “inert”CNTs and the matrix material is, therefore, essential for improvedmechanical performance of the composite.

For high-strength CNT reinforced composites, the matrix material has tobind to the CNT reinforcing filler strongly (to prevent the two surfacesfrom slipping), so that an applied load (such as a tensile stress) canbe transferred to the nanotubes. (See, for example, P. Calvert, Nature,339, 210 (1999)). Several methods, including chemical functionalizationof CNT tubule ends and side walls have been proposed and attempted toenhance bonding between CNTs and matrix material. (See, for example, J.Chen, et al. Science, 282, 95 (1998); A. Grag, et al. Chem. Phys. Lett.,295, 273 (1998), and S. Delpeux, et al. AIP Conf. Proc., 486, 470(1999)). However, no significant improvement in mechanical propertieshas been observed after such modification. Chemical coating of bothmulti-wall and single-wall CNTs with metals and metallic oxides havealso been reported for applications such as heterogeneous catalysis andone-dimensional nanoscale composites. (See, for example, T. W. Ebbesen,et al. Adv. Mater., 8, 155 (1996), X. Chen, et al. Compos. Sci.Technol., 60, 301 (2000), and L. M. Ang et al. Carbon, 38, 363 (2000)).The bonding between the coating materials and CNTs is, however, notstrong enough to result in efficient load transfer. Thus, there exists aneed in the art to improve the interaction between CNT reinforcingfillers and matrix materials in order to confer high mechanical strengthto CNT reinforced composites and enable their commercial use in themanufacture of high-strength, light-weight mechanical and electricaldevice components.

SUMMARY OF THE INVENTION

The present invention provides CNTs comprising a plurality ofmicroparticulate carbide or nitride material that provide a reinforcingeffect on the CNT matrix, thereby conferring improved mechanicalproperties in the composite materials comprising them as reinforcingfillers. In particular, the present invention provides microparticulatecarbide reinforced CNTs comprising boron carbide nanolumps formed on thesurface of CNTs. The present invention also provides a method ofproducing microparticulate carbide reinforced CNTs. Specifically, thepresent invention provides the use of microparticulate carbidereinforced CNTs having boron carbide nanolumps formed on the surface ofthe CNTs to enable their use as reinforcing composite fillers inproducing high strength composite materials.

The load transfer efficiency between a matrix material and multi-walledCNTs is increased when the inner layers of multi-walled CNTs are bondedto a matrix material. The present invention provides reinforced CNTshaving boron carbide (B_(x)C_(y)) nanolumps formed substantially on thesurface of the CNTs. The B_(x)C_(y) nanolumps reinforce CNTs by bondingnot only to the outermost layer, but also to the inner layers of theCNTs, and promote the bonding of matrix material to the inner layers ofmulti-walled CNTs. The load transfer efficiency also increasesdramatically when the shape of the CNTs allows for a greater surfacearea along the CNTs and the matrix material. Boron carbides of theformula B_(x)C_(y) are covalent bonding compounds with superiorhardness, excellent mechanical, thermal and electrical properties. Theyare therefore excellent reinforcing material for CNTs. The carbidemodified CNTs of the invention have superior mechanical properties asfillers for matrix materials, enabling the production of high-strengthcomposites.

The present invention provides the synthesis of B_(x)C_(y) nanolumps onthe surface of multi-wall CNTs. In one embodiment, present inventionuses a solid-state reaction between a boron source material andpre-formed CNTs to form boron carbide (B_(x)C_(y)) nanolumps on thesurface of CNTs. In a preferred embodiment, the B_(x)C_(y) nanolumps areformed by a solid-state reaction of magnesium diboride (MgB₂) andpre-formed CNTs. The B_(x)C_(y) nanolumps are preferably bonded to theinner layers of multi-wall CNTs. In a preferred embodiment, the bondingbetween the B_(x)C_(y) nanolumps and the CNTs is covalent chemicalbonding. Typically, such covalent chemical nanolumps bonding betweenB_(x)C_(y) and CNTs occurs in the absence of a secondary phase phaseseparation at the interface.

The present invention also provides methods of using reinforced CNTshaving B_(x)C_(y) nanolumps as reinforcing fillers in composites. Thecarbide reinforced CNTs of the invention can be used as additives toprovide improved strength and reinforcement to plastics, ceramics,rubber, concrete, epoxies, and other materials, by utilizing of standardfiber reinforcement methods for improving material strength.Additionally, the carbide reinforced CNTs comprising B_(x)C_(y)nanolumps are potentially useful for electronic applications, such asuse in electrodes, batteries, energy storage cells, sensors, capacitors,light-emitting diodes, and electrochromic displays, and are also suitedfor other applications including hydrogen storage devices,electrochemical capacitors, lithium ion batteries, high efficiency fuelcells, semiconductors, nanoelectronic components and high strengthcomposite materials. Furthermore, the methods of the present inventionprovide large scale, cost efficient synthetic processes for producinglinear and branched carbide reinforced CNTs having B_(x)C_(y) nanolumps.

The carbide-reinforced CNTs of the present invention have severaladvantages over current reinforcing materials known in the art. CNTs arereinforcing filler for composites because of their very high aspectratio, large Young's modulus, and low density. Carbide reinforced CNTsof the invention containing B_(x)C_(y) nanolumps are superiorreinforcing fillers for incorporation within a matrix material becausethe modification of carbon nanotube morphology by the B_(x)C_(y)nanolumps increases the load transfer efficiency between CNTs and thematrix material. The shape modification of CNTs by B_(x)C_(y) nanolumpsprovides a greater CNT surface area that results in stronger adhesion ofthe matrix material, while nanolump bonding to the inner layers ofmulti-wall CNTs allows for a greater load transfer from matrix materialsto CNTs. Although the carbide reinforced CNT materials of the inventionare illustrated with boron carbide (B_(x)C_(y)) as the reinforcingmaterial, it ill be understood by one skilled in the art that othermetallic and non-metallic carbides, metallic and non-metallic nitridesmay be substituted for boron carbide without departing from the scope ofthe invention. Metallic carbides, such as boron carbides, are among thehardest solids known in the art, along with diamond and boron nitride.B_(x)C_(y) has a high melting point, high modulus, low density, largeneutron capture section, superior thermal and electrical properties, andis chemically inert.

The foregoing and other aspects, features and advantages of the presentinvention will become apparent from the figures, description of thedrawings and detailed description of particular embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further explained with reference to theattached drawings, wherein like structures are referred to by likenumerals throughout the several views. The drawings shown are notnecessarily to scale, with emphasis instead generally being placed uponillustrating the principles of the present invention.

FIG. 1 shows scanning electron microscope (SEM) images of multi-wallCNTs. FIG. 1(a) shows multi-wall CNTs before the formation of B_(x)C_(y)nanolumps. FIG. 1(b) shows multi-wall CNTs after the formation ofB_(x)C_(y) nanolumps.

FIG. 2 shows transmission electron microscope (TEM) images of amulti-wall CNT with B_(x)C_(y) nanolumps. FIG. 2(a) shows a multi-wallCNT at low magnification. FIG. 2(b) shows a multi-wall CNT at mediummagnification.

FIG. 3 shows images of B_(x)C_(y) nanolumps on a multi-wall CNT. FIG.3(a) shows a high-resolution transmission electron microscope (HRTEM)image of a B_(x)C_(y) nanolump on a multi-wall carbon nanotube. FIG.3(b) shows an enlarged image of the upper portion of FIG. 3(a). FIG.3(c) shows a fast-Fourier transformation (FFT) image of FIG. 3(b). FIG.3(d) shows the twin boundaries along (101) or (01{overscore (1)}) planesof B_(x)C_(y).

FIG. 4 shows high-resolution transmission electron microscope (HRTEM)images. FIG. 4(a) shows the reacted area of a multi-wall carbonnanotube. FIG. 4(b) shows the interface between B_(x)C_(y) nanolumps anda carbon nanotube is sharp and well bonded. FIG. 4(c) shows an epitaxialrelationship between B_(x)C_(y) nanolump and a multi-wall carbonnanotube with a (101) plane perpendicular to the zigzag-type nanotubeaxis.

FIG. 5 is a schematic drawing illustrating carbon nanotube (CNT)morphologies.

FIG. 6 shows low magnification TEM photomicrographs of CNTs grown atvarying gas pressures. FIG. 6(a) shows CNTs grown at a gas pressure of0.6 torr. FIG. 6(b) shows CNTs grown at a gas pressure of 50 torr. FIG.6(c) shows CNTs grown at a gas pressure of 200 torr. FIG. 6(d) showsCNTs grown at a gas pressure of 400 torr. FIG. 6(e) shows CNTs grown ata gas pressure of 600 torr. FIG. 6(f) shows CNTs grown at a gas pressureof 760 torr.

FIG. 7 shows high magnification TEM photomicrographs of CNTs grown atvarious gas pressures. FIG. 7(a) shows CNTs grown at a gas pressure of0.6 torr. FIG. 7(b) shows CNTs grown at a gas pressure of 200 torr. FIG.7(c) shows CNTs grown at a gas pressure of 400 torr. FIG. 7(d) showsCNTs grown at a gas pressure of 760 torr.

FIG. 8 shows SEM photomicrographs of symmetrically branched (Y-shaped)CNTs. FIG. 8(a) shows symmetrically branched (Y-shaped) CNTs at lowmagnification (scale bar=1 μm). FIG. 8(b) shows symmetrically branched(Y-shaped) CNTs at high magnification (scale bar=200 nm).

FIG. 9 shows TEM photomicrographs branched CNT Y-junctions. FIG. 9(a)shows branched CNT Y-junctions with straight hollow arms and uniformdiameter (scale bar=100 nm). FIG. 9(b) shows branched CNT Y-junctionswith a pear-shaped particle cap at tubule terminal (scale bar=100 nm)(expanded in bottom inset) and XDS photomicrograph (top right inset)showing composition of particle. FIG. 9(c) shows branched CNTY-junctions shows a branched CNT with a double Y-junction (scale bar=100nm) (open tubule shown in inset). FIG. 9(d) shows branched CNTY-junctions shows a high resolution partial image of a well graphitized,hollow tubule Y-junction.

FIG. 10 shows SEM photomicrographs of CNTs grown at various gaspressures. FIG. 10(a) shows CNTs grown at a gas pressure of 0.6 torr.FIG. 10(b) shows CNTs grown at a gas pressure of 50 torr. FIG. 10(c)shows CNTs grown at a gas pressure of 200 torr. FIG. 10(d) shows CNTsgrown at a gas pressure of 400 torr. FIG. 10(e) shows CNTs grown at agas pressure of 600 torr. FIG. 10(f) shows CNTs grown at a gas pressureof 760 torr.

FIGS. 11(a-c) show low magnification TEM photomicrographs of“bamboo-like” CNTs synthesized at various temperatures. FIG. 11(a) showsCNTs synthesized at 800° C. FIG. 11(b) shows CNTs synthesized at 950° C.FIG. 11(c) shows CNTs synthesized at 1050° C. FIG. 11(d) shows CNT yielddependence on reaction temperature.

FIG. 12 shows high-resolution TEM photomicrographs of “bamboo-like” CNTssynthesized at various temperatures. FIG. 12(a) shows “bamboo-like” CNTssynthesized at 650° C. FIG. 12(b) shows “bamboo-like” CNTs synthesizedat 800° C. FIG. 12(c) shows “bamboo-like” CNTs synthesized at 1050° C.

FIG. 13 is a scanning electron micrograph (SEM) image of reinforced CNTmaterials with surface bound Magnesium oxide (MgO) showing epitaxialgrowth of MgO nanostructures on CNT tubules.

FIG. 14 shows reinforced CNT materials with surface bound amorphousboron oxide (B₂O₃) nanolumps on multi-walled CNT tubules.

While the above-identified drawings set forth preferred embodiments ofthe present invention, other embodiments of the present invention arealso contemplated, as noted in the discussion. This disclosure presentsillustrative embodiments of the present invention by way ofrepresentation and not limitation. Numerous other modifications andembodiments can be devised by those skilled in the art which fall withinthe scope and spirit of the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides CNTs comprising a plurality ofmicroparticulate carbide materials that exist substantially on the CNTsurface and function as effective reinforcing agents. Specifically, thepresent invention provides reinforced CNTs having a plurality ofmicroparticulate carbide nanolumps formed in situ on the surface of theCNTs. The present invention also provides a method of producingreinforced CNTs having B_(x)C_(y) nanolumps formed on the surface of theCNTs. The present invention also provides a method of using reinforcedCNTs having B_(x)C_(y) nanolumps formed on the surface of the CNTs asreinforcing composite fillers.

The terms “boron carbide nanolump” and “B_(x)C_(y) nanolump” refer to ananoscale aggregate comprising a boron carbide microparticles on asurface of a nanoscale carbon material, including but not, limited tocarbon nanotubes. Nanolumps are typically irregular in shape.

The term “reinforced carbon nanotube” refer to strengthened CNTs inwhich more force or effectiveness is given to the carbon nanotube. Inone embodiment of the present invention, CNTs are reinforced by reducingthe amount that the inner layers of a multi-walled CNT slip from theouter layers of the CNT. In a currently preferred embodiment, CNTs arereinforced by bonding a microparticulate carbide material substantiallyto the surface of the CNT which binds to the inner walls of the CNTs.

The term “matrix material” refers to any material capable of forming acomposite with reinforced CNTs. Examples of matrix materials include,but are not limited to, plastics, ceramics, metals, metal alloys,rubber, concrete, epoxies, glasses, polymers, graphite, and mixturesthereof. A variety of polymers, including thermoplastics and resins, canbe used to form composites with the reinforced CNT's of the presentinvention. Such polymers include, but are not limited to, polyamides,polyesters, polyethers, polyphenylenes, polysulfones, polycarbonates,polyacrylites, polyurethanes or epoxy resins.

The term “carbide forming source” refers to any suitable materialcapable of forming a carbide material. The carbide forming source can bemetallic or non-metallic. Preferred carbide forming source include, butare not limited to, magnesium diboride (MgB₂), aluminum diboride (AlB₂)calcium diboride (CaB₂), and gallium diboride (GaB₂). Preferably thecarbide forming source exists in the form of a carbide forming sourcepowder.

A “carbide material” as referred to herein is afforded the meaningtypically provided for in the art. More specifically, a carbide materialis a binary solid compound of carbon an another element. Element capableof forming carbide materials can be metallic or non-metallic. Examplesof element that can form carbides include, but are not limited to, boron(B), calcium (Ca), tungsten (W), silicon (Si), nobium (No), titanium(Ti), and iron (Fe). Carbides can have various ratios between carbon andthe element capable of forming carbide material. A presently preferredcarbide material of the present invention is boron carbide (B_(x)C_(y)).A presently preferred carbide material of the present invention is boroncarbide (B_(x)C_(y)).

The carbide materials on the surface of CNTs can be either in the formof a contiguous coating layer or a non-contiguous surface layer, suchas, for example, in the form of nanolumps. In a preferred embodiment,the carbide material is B_(x)C_(y) in a non-contiguous surface layer inthe form of nanolumps. In one embodiment, the interface betweenB_(x)C_(y) nanolumps and CNTs is sharp, in which there is no amorphouslayer in between the B_(x)C_(y) nanolumps and CNTs. The B_(x)C_(y)nanolumps may be chemically bound to the CNT surface by covalent bondingor by van der Waals type attractive forces. Preferably, the B_(x)C_(y)nanolumps are bound to CNTs covalently.

The B_(x)C_(y) nanolumps of the present invention typically have anaverage particle size from about 10 nanometers (nm) to about 200 nm.Preferably, the B_(x)C_(y) nanolumps have an average diameter of abouttwo to three times the average diameter of CNTs. In one embodiment, theB_(x)C_(y) nanolumps have an average diameter ranging from about 50 nmto about 100 nm. In a preferred embodiment, the B_(x)C_(y) nanolumpshave an average diameter of about 80 nm.

The B_(x)C_(y) lump density on the reinforced CNTs of the invention canvary over a wide range. Preferably, the nanolumps are isolatednanolumps. The spacing variation between adjacent nanolumps on a CNT canrange from about 30 nm to about 500 nm and is dependent on the particledensity on the CNT surface, which is expressed as a ratio of thepercentage of boron atoms to carbon atoms in the boron carbideB_(x)C_(y) (atom % carbon). In a preferred embodiment, the spacingbetween B_(x)C_(y) nanolumps is from about 50 nm to about 100 nm.

The B_(x)C_(y) nanolumps in the reinforced CNTs of the present inventioncan be crystalline or amorphous. In a preferred embodiment, theB_(x)C_(y) nanolumps are crystalline. The crystal geometries of theB_(x)C_(y) nanolumps include, but are not limited to, rhombohedral,tetragonal and orthorhombic. In a preferred embodiment, the crystalstructure of the B_(x)C_(y) nanolumps is rhombohedral.

The ratio of boron to carbon in the B_(x)C_(y) nanolumps is variable.Boron carbides typically exists as stable single phase, with ahomogeneity ranging from about 8 atom % carbon to about 20 atom %carbon. Examples of boron carbon ratios within this range are B₄C andB₁₀C. The boron carbide nanolumps in the reinforced CNTs of theinvention have the general formulas B_(x)C_(y) wherein x ranges from4-50 and y ranges form 1-4. The most stable B_(x)C_(y) structures arerhombohedral with a stoichiometry of B₁₃C, B₁₂C₃ or B₄C, tetragonal witha stoichiometry of B₅₀C₂, B₅₀C, B₄₈C₃, B₅₁C, B₄₉C₃, or orthorhombic witha stoichiometry of B₈C. Other stable B_(x)C_(y) structures include B₁₂C,B₁₂C₂ and B₁₁C₄. In one embodiment, the ratio of boron to carbon is 4boron atoms to one carbon atom (B₄C).

Typically, twin boundaries can be observed in B₄C nanolumps. In oneembodiment, the twin boundary is along either (101) or (01{overscore(1)}) planes, as shown in FIG. 3(d).

FIG. 3 shows images of B_(x)C_(y) nanolumps on a multi-wall CNTs. FIG.3(a) shows an HRTEM image of a B_(x)C_(y) nanolump on a multi-wallcarbon nanotube. FIG. 3(b) shows an enlarged image of the upper portionof FIG. 3(a). FIG. 3(c) shows a FFT image of FIG. 3(b). The simulatedimage, as shown in the inset of FIG. 3(b), and the indexing of the FFTimage, as shown in FIG. 3(c), were carried out by using structuralparameters of B_(x)C_(y) and zone axis of ({overscore (1)}11). FIG. 3(d)shows the twin boundaries along (101) or (01{overscore (1)}) planes ofB_(x)C_(y). The main parameters for the simulated image, as shown in theinset of FIG. 3(b), are: spherical aberration coefficient=0.5 mm,thickness=10 nm, and defocus=50 nm.

B_(x)C_(y) nanolumps of the invention provide materials such as carbonfibers and CNTs with a knotted-rope-shaped or bone-shaped morphology.Knotted-rope-shaped CNTs and bone-shaped CNTs can be excellentreinforcing fillers to increase strength and toughness due to a moreeffective load transfer between CNTs and matrix materials. The lumps orknots allow for mechanical matrix-CNT interlocking.

Another aspect of the present invention is a method of producing CNTshaving boron carbide (B_(x)C_(y)) nanolumps formed on the surface of theCNTs. The method of the present invention can be applied to CNTscomprising any morphology including aligned or non-aligned lineararrays. Preferably, the CNTs have a branched, multi-walled morphology.

The term “carbide forming source” refers to metallic or non-metallicmaterial, known in the art, capable of forming a carbide in-situ on theCNT surface. Examples of a carbide forming source include, but are notlimited to, Magnesium diboride (Mg B₂), aluminum diboride (AlB₂),calcium diboride (CaB₂) and gallium diboride (GaB₂). A preferred carbideforming source is Magnesium diboride (Mg B₂).

B_(x)C_(y) nanolumps can be grown on CNTs using any suitable method. Inone embodiment, B_(x)C_(y) nanolumps are grown on CNTs by using a solidstate reaction between a boron source and CNTs. Any suitable boronsource known in the art can be used. Suitable boron sources include, butare not limited to, magnesium diboride (MgB₂) and aluminum diboride(AlB₂). In a preferred embodiment, the boron source is MgB₂. Preferably,the boron source is in the form of a powder. In one embodiment, thepowder comprises particles with an average grain size of about 0.1micrometer (μm) to about 100 micrometers (μm). Preferably, the powdercomprises particles with an average grain size of about 1 micrometer.The synthesis of magnesium diboride (MgB₂) powders is accomplished bycombining elemental magnesium and isotopicaly pure boron by knownmethods.

The boron source used in the present invention decomposes at atemperature of between about 100° C. to about 1000° C., preferably, at atemperature of about 600° C. Thermally decomposed boron is typicallymore reactive chemically; the solid-state reaction can, therefore, beperformed at relatively low temperatures. In one embodiment, the solidstate reaction is performed at temperatures ranging from about 500° C.to about 2000° C. In a preferred embodiment, the solid state reaction isperformed at temperature of ranging from about 1000° C. to about 1250°C.

The CNTs used for producing reinforced CNTs of the present invention arepurified by any suitable method known in the art prior to introductionof B_(x)C_(y) nanolumps. In one embodiment, CNTs are purified by washingwith a mineral acid. Examples of suitable mineral acids include, but arenot limited to, hydrofluoric acid (HF), hydrochloric acid (HCl),hydrobromic acid (HBr), hydroiodic acid (HI), sulfuric acid (H₂SO₄) ornitric acid (HNO₃). In a preferred embodiment, the mineral acid is HCland HNO₃.

The purified CNTs nanotubes are then be mixed with the boron sourcepowder. Gentle mechanical mixing following which the mixture is wrappedwith a metal foil to form an assembly. Preferred metal foils include,but are not limited to, transition metal foils. In a currently preferredembodiment, the metal foil is Tantalum (Ta). The assembly is then placedin a ceramic tube furnace, in a vacuum of about 0.5 torr by mechanicalpump. In one embodiment, the reaction area is localized only at the areawhere boron is present. That is, no surface diffusion of boron isobserved in the solid-state reaction.

Alternate methods for the formation of B_(x)C_(y) nanolumps such aschemical vapor deposition (CVD) can be used. In one embodiment of thepresent invention, CVD of boron carbide such as plasma enhanced chemicalvapor deposition (PECVD), hot filament chemical vapor deposition(HFCVD), and synchrotron radiation chemical vapor deposition (SRCVD)using reactive gas mixtures such as BCl₃—CH₄—H₂, B₂H₆—CH₄—H₂, B₅H₉—CH₄,BBr₃—CH₄—H₂, C₂B₁₀H₁₂, BCl₃—C₇H₈—H₂, B(CH₃)₃ and B(C₂H₅)₃ are used. Oneembodiment, of the present invention uses a solid state reaction betweena carbide forming source and CNTs. Another embodiment, of the presentinvention uses a solid state reaction between a boron source and CNTs.

The present invention provides a method of manufacturing reinforcedcarbon nanotubes having a plurality of boron carbide nanolumps formedsubstantially on a surface of pre-formed CNTs comprising the steps of:(1) purifying a plurality of carbon nanotubes by washing with a mineralacid; (2) mixing the plurality of carbon nanotubes with a boron sourcepowder to form a mixture of carbon nanotubes and boron source powder;(3) wrapping the mixture of carbon nanotubes and boron source powderwithin a metal foil; (4) placing the metal foil containing the mixtureof carbon nanotubes and boron source powder in a ceramic tube furnace;(5) pumping the ceramic tube furnace to below about 0.5 torr by amechanical pump; and (6) heating the ceramic tube furnace.

In one aspect of the present invention, a material comprising aplurality of reinforced carbon nanotubes having a plurality of boroncarbide nanolumps formed substantially on the surface of the CNTs isused as reinforcing fillers for materials comprising the step ofcombining the plurality of reinforced carbon nanotubes and a matrixmaterial to form a high-strength composite.

FIG. 1(a) shows a SEM image of the CNTs before the growth of boroncarbide nanolumps. FIG. 1(b) shows a SEM image of B_(x)C_(y) nanolumpson the surface of multi-wall carbon nanotubes. The B_(x)C_(y) nanolumpsform into a desired morphology, individual nanoparticles instead of ahomogeneous layer on the surface of multi-wall carbon nanotubes. Theaverage particle size of the B_(x)C_(y) nanolumps is about 80 nm indiameter, which is two or three times of the average diameter of CNTs.The lump density on a carbon nanotube varies dramatically, with aspacing variation between adjacent nanolumps from about 30 nm to about500 nm.

FIG. 2(a) and FIG. 2(b) show TEM images of B_(x)C_(y) nanolumps onmulti-wall CNTs at low and medium magnifications, respectively. Theaverage particle size shown in FIG. 2(a) is about 50 nm, smaller thanthat shown in FIG. 2(b). As shown in FIG. 2(a) and FIG. 2(b), thereaction between boron and CNTs is confined and the main structure ofmulti-wall CNTs remains unchanged. X-ray energy dispersive spectrometer(EDS) analysis on the composition of the nanolumps shows that thenanolumps contain only carbon. No magnesium (Mg) or Boron (B) weredetected. The Mg from the decomposition of magnesium diboride (MgB₂)becomes vapor at the reaction temperature of about 1100° C. to about1150° C. and was pumped out. But the existence of boron can not beexcluded because boron can not be detected by the EDS system, since thelow energy x-rays from boron atoms were absorbed by detector.

FIG. 4(a) shows an interface between B_(x)C_(y) nanolump and multi-wallcarbon nanotube. Part of multi-wall CNTs is reacted with boron by asolid state reaction, therefore no lattice fringes of CNTs can beobserved at the bottom portion of the B_(x)C_(y) nanolump. The solidstate reaction area is localized only at the area where there is boron.No surface diffusion of boron is observed in the solid-state reaction.As shown by the HRTEM images of FIG. 4(a) and FIG. 4(b), the interfacebetween B_(x)C_(y) nanolumps and CNTs is sharp. No amorphous layer wasfound at the interface between B_(x)C_(y) nanolumps and CNTs. Anepitaxial relationship between CNTs and B_(x)C_(y) nanolumps is shown inFIG. 4(c) and supports the conclusion of strong interface betweenB_(x)C_(y) nanolumps and CNTs. Inner layers of CNTs at the reaction areaare also bonded to B_(x)C_(y) as shown in FIG. 4(a) and FIG. 4(b). Thebonding between B_(x)C_(y) nanolumps and CNTs is strong, most likely, acovalent bonding, because the bonding between boron atoms and carbonatoms inside B_(x)C_(y) is covalent.

The strong bonding at the interface between B_(x)C_(y) nanolumps andCNTs can prevent the breaking at the interface between B_(x)C_(y)nanolumps and CNTs during load transfer. Bone-shaped short fibers werereported to be ideal reinforcing fillers to increase strength andtoughness due to a more effective load transfer. Therefore, themodification of CNT morphology by B_(x)C_(y) nanolumps increases theload transfer between nanotubes and matrix. Moreover, inner layers ofmulti-wall CNTs are also bonded to B_(x)C_(y) nanolumps, so the innerlayers can also contribute to load carrying, instead of only the outmostlayer.

Reinforced CNTs can be used to form or reinforce composites with othermaterials, especially a dissimilar material. Suitable dissimilarmaterials include, but are not limited to, metals, ceramics, glasses,polymers, graphite, and mixtures thereof. Such composites may beprepared, for example, by coating the reinforced CNTs with thedissimilar material either in a solid particulate form or in a liquidform. A variety of polymers, which include but are not limited to,thermoplastics and resins can be utilized to form composites with theproducts of the present invention. Such polymers include, but are notlimited to, polyamides, polyesters, polyethers, polyphenylenes,polysulfones, polyurethanes or epoxy resins. Branched CNTs of thepresent invention can find application in construction of nanoelectronicdevices and in fiber-reinforced composites. The Y-junction CNT fibers ofthe invention are expected to provide superior reinforcement tocomposites compared to linear CNTs.

The carbon nanotubes comprised in the reinforced CNTs of the presentinvention can possess any of the several known morphologies. Examples ofknown CNT morphologies include, but are not limited to, linear,non-linear, branched, “bamboo-like”, and non-linear(“spaghetti-shaped”). Individual tubules of such CNTs can be eithersingle or multi-walled. CNTs with the above morphologies are described,for example, in Li, et al., Appl. Phys. A: Mater. Sci. Process, 73, 259(2001) and U.S. application Ser. No. 10/151,382, filed on May 20, 2002.Both references are hereby incorporated herein by reference in theirentirety. In a currently preferred embodiment, the reinforced CNTs ofthe invention have a branched, multi-walled tubule morphology.

The CNTs in the carbide reinforced CNT materials of the presentinvention can be aligned or non-aligned. Preferably, the CNTs arenon-aligned, substantially linear, concentric tubules with hollow cores,or capped conical tubules stacked in a bamboo-like arrangement.Referring to FIG. 5, the nanotube morphology can be controlled bychoosing an appropriate catalyst material and reaction conditions.Depending on the choice of reaction conditions, relatively largequantities (kilogram scale) of morphologically controlled CNTssubstantially free of impurity related defects, such as for example,from entrapment of amorphous carbon or catalyst particles, can beobtained. The linear CNTs obtained by the methods of the presentinvention have diameters ranging from about 0.7 nanometers (nm) to about200 nanometers (nm) and are comprised of a single graphene layer or aplurality of concentric graphene layers (graphitized carbon). Thenanotube diameter and graphene layer arrangement may be controlled byoptimization of reaction temperature during the nanotube synthesis.

FIG. 6 shows low magnification TEM images of linear CNTs grown at low,intermediate and high gas pressures. The low magnification TEM images oflinear CNTs of FIG. 6 are indicative that tubule morphology can becontrollably changed by choice of gas pressure “feeding” into a reactorfor CNT preparation. The control of gas pressures in the methods of thepresent invention is accomplished by regulating gas pressure of thegases feeding in to the reactor using conventional pressure regulatordevices. FIG. 6(a) shows CNTs grown at a gas pressure of about 0.6 torr.CNTs grown at a gas pressure of about 0.6 torr predominantly have amorphology that consists of a tubular configuration, completely hollowcores, small diameter, and a smooth surface. FIG. 6(b) shows CNTs grownat a gas pressure of about 50 torr. CNTs grown at a gas pressure ofabout 50 torr have a morphology that is essentially similar to that atabout 0.6 torr, except that a small amount of tubules have an end cappedconically shaped stacked configuration (“bamboo-like”). FIG. 6 c showsCNTs grown at a gas pressure of about 200 torr. The CNTs grown at a gaspressure of about 200 torr have a morphology of predominantly theend-capped, conical stacked configurations (“bamboo-like”) regardless ofthe outer diameters and wall thickness of the CNTs. As shown in FIG.6(c), the density of the compartments within individual tubules of theCNTs is high, with inter-compartmental distance inside the “bamboo-like”structures ranging from about 25 nm to about 80 nm.

At gas pressures greater than about 200 torr, an entirely “bamboo-like”morphology is obtained for the CNTs, with increased compartmentaldensity. The inter-compartmental distances within the individual CNTsdecrease with increasing gas pressure (about 10 nm to about 50 nm atabout 400 torr and about 10 nm to about 40 nm at about 600 torr andabout 760 torr, respectively). As shown in FIG. 6(f), CNTs synthesizedat about 760 torr have a wider tubule diameter of about 20 nm to about55 nm. CNTs synthesized at about 760 torr have thin walls and smoothsurfaces. In comparison to linear CNTs synthesized at a gas pressure ofabout 200 torr, CNTs synthesized at higher pressures of about 400 torrand about 600 torr are highly curved and have broken ends, as shown inFIG. 6(d) and FIG. 6(e). The highly curved and broken ends areattributed to fracturing of the CNTs during the TEM specimenpreparation, which is indicative that CNTs with a “bamboo-like”morphology may be readily cleaved into shorter sections compared to thetubular type.

CNTs of the present invention have a relatively high degree ofgraphitization (process of forming a planar graphite structure or“graphene” layer). The complete formation of cytstalline graphenelayers, and the formation of multiple concentric layers within eachtubule and hollow core, with minimal defects (such as defects typicallycaused by entrapment of non-graphitized, amorphous carbon and metalcatalyst particles) is an important prerequisite for superior mechanicalproperties in CNTs.

FIG. 7 shows TEM photomicrographs detailing morphologies of linear CNTsgrown at different gas pressures. As shown in FIG. 7, CNTs grown atpressures between about 0.6 torr to about 200 torr have goodgraphitization, in which the walls of the CNTs comprise about 10graphene layers which terminate at the end of the CNT that is distalfrom the substrate (i.e., the fringes are parallel to the axis of theCNT), and possess completely hollow cores. Linear CNTs grown at about200 torr have tubule walls comprising about 15 graphene layers.Individual tubules are “bamboo-like” rather than completely hollow, withdiaphragms that contain a low number (less than about 5) of graphenelayers. Graphene layers terminate at the surface of the CNTs, with theangle between the fringes of the wall and the axis of the CNT (theinclination angle) being about 3°, as shown in FIG. 7(b). FIG. 7(c)shows linear CNTs grown at intermediate gas pressures (about 400 torr toabout 600 torr) have a “bamboo-like” structure. A “bamboo-like”structure typically has more of graphene layers in the walls anddiaphragms of tubules (typically about 25 and about 10 graphene layersin the CNT walls and diaphragms, respectively), but less graphitization(lower crystallinity) due to a faster growth rate. Despite the lowcrystallinity, graphene layers terminate on the tubule surface withinclination angle of about 6°. As shown in FIG. 7(d), CNTs grown atabout 760 torr have higher graphitization than CNTs grown at about 400torr to about 600 torr, have a “bamboo-like” structural morphologyconsisting of parabolic-shaped layers stacked regularly along thesymmetric axes of the CNTs. The graphene layers terminate within a shortlength along growth direction on the surface of the CNTs resulting in ahigh density of exposed edges for individual layers. As shown in FIG.7(d), the inclination angle of the fringes on the wall of the CNTs isabout 13°. The high number of terminal carbon atoms on the tubulesurface is expected to impart differentiated chemical and mechanicalproperties in the CNTs compared the hollow, tubular type, and render theCNTs more amenable for attachment of organic molecules.

CNTs can comprise a branched (“Y-shaped”) morphology, referred to hereinas “branched CNTs”, wherein the individual arms constituting branchedtubules are either symmetrical or unsymmetrical with respect to both armlengths and the angle between adjacent arms. In one embodiment, theY-shaped CNTs exist as (1) a plurality of free standing, branched CNTsattached to the substrate and extending outwardly from the substrateouter surface; and (2) one or more CNTs with a branched morphologywherein the CNT tubule structures have Y-junctions with substantiallystraight tubular arms and substantially fixed angles between said arms.

As seen in FIG. 8, branched CNTs can comprise a plurality of Y-junctionswith substantially straight arms extending linearly from said junctions.The majority of branched CNTs possess Y-junctions having two long armsthat are a few microns long (about 2 μm to about 10 μm), and a third armthat is shorter (about 0.01 μm to about 2 μm). CNTs with Y-junctionscomprising three long arms (up to about 10 μm), and with multiplebranching forming multiple Y-junctions with substantially linear,straight arms can be also obtained by the method of the invention. Asshown in FIG. 8(b), a high magnification SEM micrograph shows that thebranched CNTs of the invention possess Y-junctions that have a smoothsurface and uniform tubule diameter about 2000 nm. The angles betweenadjacent arms are close to about 120°, thereby resulting in branchedCNTs that have a substantially symmetric structure. Y-junctions have asubstantially similar structural configuration, regardless of thevarying tubule diameters of the CNTs.

As shown in FIG. 9, Y-junctions of branched CNTs have hollow coreswithin the tubular arms of branched CNTs. As shown in the inset of FIG.9(a), a triangular, amorphous particle is frequently found at the centerof the Y-junction. Compositional analysis by an x-ray energy dispersivespectrometer (EDS) indicates that the triangular particles consist ofcalcium (Ca), silicon (Si), magnesium (Mg), and oxygen (O). The calcium(Ca) and silicon (Si) are probably initially contained in the catalystmaterial. It is frequently observed that one of the two long arms of theY-junction is capped with a pear-shaped particle (FIG. 9(b) and lowerinset), that a similar chemical composition as that of theaforementioned triangle-shaped particle found within the tubules at theY-junction. A trace amount of cobalt (Co) from the catalytic material isfound at the surface of such pear-shaped particle. FIG. 9(b) shows thatthe tubule of the other long arm of the branched CNT is filled withcrystalline magnesium oxide (MgO) from the catalytic material (confirmedby diffraction contrast image in the EDS spectrograph). The upper rightinset in FIG. 9(b) shows selected area diffraction patterns, whichindicate that one of the (110) reflections, (101), of the magnesiumoxide (MgO) rod is parallel to (0002) reflection (indicated by arrowheads) from carbon nanotube walls. Therefore, the magnesium oxide (MgO)rod axis is along (210). Additionally, Y-junctions filled withcontinuous single crystalline magnesium oxide (MgO) from one arm, acrossa joint, to another arm can also be obtained. FIG. 12(c) shows a doubleY-junction, wherein only one arm of the right-side Y-junction is filledwith single crystal MgO. The inset of FIG. 12(b) shows a magnified imageof the end of the MgO filled arm, illustrating an open tip that providesentry of MgO into the CNT Y-junctions. FIG. 12(d) shows a highlymagnified partial Y-junction, which is well graphitized, and consists ofabout 60 concentric graphite layers (partially shown) in its tubulearms, and a hollow core with a diameter of about 8.5 nm. CNTs cancomprise a plurality of free standing, linearly extending carbonnanotubes originating from and attached to the surface of a catalyticsubstrate having a micro-particulate, mesoporous structure with particlesize ranging from about 0.1 μm to about 100 μm, and extending outwardlyfrom the substrate outer surface. The morphology of individual CNTtubules can either be cylindrical with a hollow core, or be end-capped,stacked and conical (“bamboo-like”). Both morphological forms may becomprised of either a single layer or multiple layers of graphitizedcarbon. CNTs can also be separated from the catalytic substrate materialand exist in a free-standing, unsupported form.

In another embodiment of the present invention, the reinforced CNTmaterial comprises a microparticulate oxide material that are boundsubstantially on the surface of the CNT tubules. The microparticulateoxide materials of the invention can be metallic or non-metallic oxides.Examples of oxide materials include, but are not limited to, magnesiumoxide (MgO) and boron oxide (B₂O₃). As shown in FIG. 14 amorphous boronoxide (B₂O₃) nanolumps are formed on multi-walled CNTs. FIG. 14(a) showsa scale bar equal to 100 nanometers. FIG. 14(b) shows a scale bar equalto 200 nanometers. FIG. 14(c) shows a scale bar equal to 10 nanometers.

CNTs can be grown by any suitable method known in the art. For example,multi-wall CNTs can be grown by any CVD method, including but notlimited to, plasma enhanced chemical vapor deposition (PECVE), hotfilament chemical vapor deposition (HFCVD), or synchrotron radiationchemical vapor deposition (SRCVD). Suitable methods for growing CNTs aredescribed by Li, et al., Appl. Phys. A: Mater. Sci. Process, 73, 259(2001) and U.S. application Ser. No. 10/151,382, filed on May 20, 2002,the contents of both these references are hereby incorporated herein byreference in their entireties.

EXAMPLES Example 1

Synthesis of Reinforced CNTs Having Boron Carbide (B_(x)C_(y) )Nanolumps Formed Substantially on the Surface of the CNTs

The multi-wall CNTs were grown by catalytic chemical vapor depositionmethod (see Li, et al., Appl. Phys. A: Mater. Sci. Process, 73, 259(2001), the contents of which is incorporated herein by reference in itsentirety) and purified by hydrofluoric acid (HF). Magnesium diboride(MgB₂), a new superconducting material, is used as the source of boron.The synthesis of magnesium diboride (MgB₂) can be synthesised bycombining elemental magnesium and boron in a sealed (Ta) tube in astoichiometric ratio and sealed in a quartz ampule, placed in a boxfurnace at a temperature of about 950° C. for about 2 hours. Powder MgB₂with average grain size of about 1 micrometer decomposes at atemperature of about 600° C. Thermally decomposed boron is morechemically reactive so the solid-state reaction can be performed atrelatively low temperatures. The nanotubes were mixed gently with MgB₂powder first, then wrapped by a tantalum (Ta) foil to form an assembly,and finally the assembly was placed in a ceramic tube furnace, andpumped to below about 0.5 torr by mechanical pump. The sample was heatedat about 1100° C. to about 1150° C. for about 2 hours. Microstructuralstudies were carried out by a JEOL JSM-6340F scanning electronmicroscope (SEM) and JEOL 2010 transmission electron microscope (TEM),respectively. The TEM is equipped with an X-rays energy dispersivespectrometer (EDS). A TEM specimen was prepared by dispersing CNTs intoan acetone solution by sonication and then putting a drop of thesolution on a holey carbon grid.

Example 2

Determining the Composition of B_(x)C_(y) Nanolumps

In order to find out whether the nanolumps are boron carbide, ahigh-resolution transmission electron microscopic (HRTEM) image of ananolump is taken and shown in FIG. 3(a). The carbon nanotube nature hasbeen preserved after the reaction. The B_(x)C_(y) nanolump iscrystalline. FIG. 3(b) is an enlarged HRTEM image of the top part ofFIG. 3(a). FIG. 3(c) shows a fast-Fourier transformation (FFT) image ofthe HRTEM image shown in FIG. 3(b). The diffraction pattern obtainedfrom FFT (FIG. 3(c)) is indexed as one from zone axis ({overscore(1)}11) of B₄C. Structure parameters of B₄C for the indexing are spacegroup R3m: (166) and lattice parameters, a=0.56 nm, c=1.21 nm. As shownin FIG. 3(b), the simulated HRTEM image using parameters defocus −30 nmand thickness 20 nm also matches with experimental image very well.Although no boron was detected by the EDS analysis, it is reasonable todraw a conclusion that the nanolumps are of the formula, B_(x)C_(y),since both calculated HRTEM image and diffraction pattern match withexperimental ones very well when using structural parameters of B₄C. Theratio between boron and carbon in nanolumps may differ from B₄Cdramatically because boron and carbon atoms can easily substitute eachother. Twin boundaries were often observed in B₄C nanolumps. As shown inFIG. 3(d), the twin boundary is along either (101) or (01{overscore(1)}) planes.

Example 3

Preparation of Catalyst Substrate for Synthesis of Linear CNTs

Mesoporous silica containing iron nanoparticles were prepared by asol-gel process by hydrolysis of tetraethoxysilane (TEOS) in thepresence of iron nitrate in aqueous solution following the methoddescribed by Li et al. (Science, (1996), Vol. 274, 1701-3) with thefollowing modification. The catalyst gel was dried to remove excesswater and solvents and calcined for about 10 hours at about 450° C. andabout 10⁻² torr to give a silica network with substantially uniformpores containing iron oxide nanoparticles that are distributed within.The catalyst gel is then ground into a fine, micro-particulate powdereither mechanically using a ball mill or manually with a pestle andmortar. The ground catalyst particles provide particle sizes that rangebetween about 0.1 μm and about 100 μm under the grinding conditions.

Example 4

Preparation of Catalyst Substrate for Synthesis of Branched CNTs

Magnesium oxide (MgO) supported cobalt (Co) catalysts were prepared bydissolving about 0.246 g of cobalt nitrate hexahydrate (Co(NO₃)₂.6H₂O,98%) in 40 ml ethyl alcohol, following which immersing about 2 g ofparticulate MgO powder (−325 mesh) were added to the solution withsonication for about 50 minutes. The solid residue was filtered, driedand calcined at about 130° C. for about 14 hours.

Example 5

General Synthetic Procedure for Linear CNTs

The synthesis of CNTs is carried out in a quartz tube reactor of achemical vapor deposition (CVD) apparatus. For each synthetic run, about100 mg of the micro-particulate catalyst substrate was spread onto amolybdenum boat (about 40×100 mm²) either mechanically with a spreaderor by spraying. The reactor chamber was then evacuated to about 10⁻²torr, following which the temperature of the chamber was raised to about750° C. Gaseous ammonia was introduced into the chamber at a flow rateof about 80 sccm and maintained for about 10 minutes, following whichacetylene at a flow rate of about 20 sccm was introduced for initiateCNT growth. The total gas pressure within the reaction chamber wasmaintained at a fixed value that ranged from about 0.6 torr to about 760torr (depending on desired morphology for the CNTs). The reaction timewas maintained constant at about 2 hours for each run. The catalyticsubstrate containing attached CNTs were washed with hydrofluoric acid,dried and weighed prior to characterization.

Example 6

General Synthetic Procedure for Branched CNTs

The MgO supported cobalt catalyst of Example 3 were first reduced atabout 1000° C. for about 1 hour in a pyrolytic chamber under a flow of amixture hydrogen (about 40 sccm) and nitrogen (about 100 sccm) at apressure of about 200 Torr. The nitrogen gas was subsequently replacedwith methane (about 10 sccm) to initiate CNT growth. The optimumreaction time for producing branched CNTs was about 1 hour.

Example 7

Characterization of CNT Morphology and Purity by Scanning ElectronMicroscopy (SEM), and Tubule Structure and Diameter by TransmissionElectron Microscopy (TEM)

Scanning electron microscopy (SEM) for characterization and verificationof CNT morphology and purity was performed on a JEOL JSM-6340Fspectrophotometer that was equipped with an energy dispersive x-ray(EDS) accessory. Standard sample preparation and analytical methods wereused for the SEM characterization using a JEOL JSM-6340F microscope. SEMmicrographs of appropriate magnification were obtained to verify tubulemorphology, distribution and purity.

Transmission electron microscopy (TEM) to characterize individual tubulestructure and diameter of the CNTs was performed on a JEOL 2010 TEMmicroscope. Sample specimens for TEM analysis were prepared by mildgrinding the CNTs in anhydrous ethanol. A few drops of the groundsuspension were placed on a micro-grid covered with a perforated carbonthin film. Analysis was carried out on a JEOL 2010 microscope. TEMmicrographs of appropriate magnification were obtained for determinationof tubule structure and diameter.

Example 7

Synthetic Procedure for Oxide Reinforced CNTs

Reinforced CNT materials comprising microparticulate oxide are obtainedin a manner substantially similar to the procedure described inExample 1. The oxide source materials used are magnesium oxide (MgO) andboron oxide (B₂O₃). The microparticulate oxide formation on CNTs iscarried out a pressure of 5 torr.

Although the examples described herein have been used to describe thepresent invention in detail, it is understood that such detail is solelyfor this purpose, and variations can be made therein by those skilled inthe art without departing from the spirit and scope of the invention.

All patents, patent applications, and published references cited hereinare hereby incorporated herein by reference in their entirety. Whilethis invention has been particularly shown and described with referencesto preferred embodiments thereof, it will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the scope of the invention encompassed by theappended claims.

1. A multi-walled carbon nanotube material having an outermost layer and at least one inner layer comprising a microparticulate carbide or oxide material wherein said microparticulate carbide or oxide material is bonded to the outermost layer and to at least one inner layer of said multi-walled carbon nanotube material.
 2. The multi-walled carbon nanotube material of claim 1, wherein the microparticulate carbide or oxide material are carbide or oxide nanoparticles.
 3. The multi-walled carbon nanotube material of claim 1, wherein the microparticulate carbide or oxide material exists substantially as carbide or oxide nanolumps on the outermost layer of the carbon nanotube material.
 4. The multi-walled carbon nanotube material of claim 3, wherein the carbide or oxide nanolumps have an average diameter ranging from 10 to 200 nanometers.
 5. The multi-walled carbon nanotube material of claim 3, wherein the carbide or oxide nanolumps have an average diameter of about 80 nanometers.
 6. The multi-walled carbon nanotube material of claim 3, wherein the carbide or oxide nanolumps reside proximally to one another and remain bound to the outermost layer and to at least one inner layer of said multi-walled carbon nanotube material by physical or chemical bonding.
 7. The multi-walled carbon nanotube material of claim 6, wherein the carbide or oxide nanolumps have an inter-particle spacing ranging from 30 to 500 nanometers.
 8. The multi-walled carbon nanotube material of claim 6, wherein the carbide or oxide nanolumps have an inter-particle spacing ranging from 50 to 100 nanometers.
 9. The multi-walled carbon nanotube material of claim 1, wherein the microparticulate carbide material is a metallic or a non-metallic carbide.
 10. The multi-walled carbon nanotube material of claim 9, wherein the microparticulate carbide material is chosen from the group consisting of boron carbide, silicon carbide, magnesium carbide, titanium carbide, and nobium carbide.
 11. The multi-walled carbon nanotube material of claim 9, wherein the microparticulate material is boron carbide.
 12. The multi-walled carbon nanotube material of claim 11, wherein the boron carbide has the formula B_(x)C_(y), wherein x is 4 to 50 and y is 1 to
 4. 13. The multi-walled carbon nanotube material of claim 12, wherein the boron carbide (B_(x)C_(y)) has a stoichiometry selected from the group consisting of B₄C, B₁₀C, B₁₃C, B₁₂C₃, B₅₀C₂, B₅₀C, B₄₈C₃, B₅₁C, B₄₉C₃, B₈C, B₁₂C, B₁₂C₂ and B₁₁C₄.
 14. The multi-walled carbon nanotube material of claim 12, wherein the boron carbide has the formula B₄C.
 15. The multi-walled carbon nanotube material of claim 1, wherein the oxide material is a metallic or non-metallic oxide.
 16. The multi-walled carbon nanotube material of claim 15, wherein the oxide material is magnesium oxide (MgO) or boron oxide (B₂O₃).
 17. The multi-walled carbon nanotube material of claim 9 wherein a microparticulate carbide material is covalently bonded to the multi-walled carbon nanotube material.
 18. The multi-walled carbon nanotube material of claim 1, wherein the microparticulate carbide material exists as a stable single phase in a homogeneity ranging from 8 to 20 atom % carbon.
 19. The multi-walled carbon nanotube material of claim 1, wherein the carbon nanotubes material have a knotted rope-shaped morphology.
 20. The multi-walled carbon nanotube material of claim 1, wherein the carbon nanotubes have a bone-shaped morphology.
 21. A composite material comprising a multi-walled carbon nanotube material and a matrix material, said multi-walled carbon nanotube material having an outermost layer and at least one inner layer comprising a microparticulate carbide or oxide material wherein said microparticulate carbide or oxide material is bonded to the outermost layer and to at least one inner layer of said multi-walled carbon nanotube material.
 22. The composite material of claim 21, wherein the microparticulate carbide material is boron carbide. 